The opening of the Ligurian Sea seen through combined deep seismic, gravity and kinematic analyses

The Western Mediterranean is constituted by a series of back-arc basins that opened in response to the African slab rollback throughout the Alpine orogenies. The Ligurian Basin occupies the northeastern termination of this realm and resulted from the rifting and subsequent drift of the Corsica-Sardinia block, between Oligocene and Burdigalian-Langhian times, coevally with the neighbouring Western Alpine collision. The nature of its basement, beneath thick sedimentary deposits, has long remained elusive. The SEFASILS cruise acquired deep penetrating wide angle seismic data from densely deployed ocean bottom sensors, as well as long offset reflection and gravity data. The obtained tomographic images unequivocally reveal a large expanse of exhumed mantle flooring the northern half of the basin beneath basinal deposits. Further south, intermediate crustal velocities are found and the nature of the basement is more ambiguous. Using satellite-derived gravity measurements and taking into account the documented kinematics of the main phase of the Ligurian opening, we show that most of the seafloor—if not all—is indeed of oceanic origin and that the observed mantle tract was emplaced from an accretion centre inside the basin rather than from under the flanking margin. In particular, the extinct spreading axis is revealed by free-air gravity anomalies. These results thus show that, albeit significant opening rates of ~4 cm/yr or more are inferred here, seafloor spreading consisted essentially in mantle unroofing with little to no melt production. Moreover, a domain of ultrathinned continental crust is also evidenced at the toe of the northern margin, that is evocative of some ductile-dominant deformation immediately prior to breakup. Mantle exhumation seems to have occurred successively and somewhat continuously throughout the basin formation on opposite-verging continental and oceanic detachment systems, active prior to and after breakup respectively.